Acoustic surface wave device

ABSTRACT

An acoustic surface wave device using an interdigital electrode array 2, 3 to launch and receive surface waves overcomes problems of diffraction by making the arrays approximately 3λ c  wide between the outer boundaries of the bus bars. As a result, the arrays can each only propagate and transduce a single acoustic surface waveguide mode which is symmetrical about the axis of the array.

This invention relates to an acoustic surface wave device including apiezoelectric substrate for propagating acoustic surface waves at asurface thereof and an acoustic surface wave transducer formed on saidsurface to launch or to receive acoustic surface waves at said surfacealong a propagation direction parallel to the axis of the transducer, inwhich said transducer includes at least one interdigital array of twooverlapping sets of electrodes and each set is connected to one of apair of opposite bus bars. Such a device will be referred to herein asan acoustic surface wave device of the kind referred to.

Such an acoustic surface wave device can comprise a filter arrangementas illustrated for example in FIG. 3.7 of page 71 of Acoustic SurfaceWaves edited by A. A. Oliner published 1978 by Springer-Verlag, in whicha uniform transducer is coupled directly to an apodized transducer.

An acoustic surface wave filter of this kind can be computed which,under ideal conditions, will satisfy a desired pass band response towithin given approximations, by techniques employing Fourier synthesisand computer optimization. Thus computer programs have been devisedwhich will specify the required distribution and intensities ofidealized transducing elements (sources) along the transducer. In anapodized interdigital array, such elements are realized in practice bythe overlap of adjacent electrodes one from each of two overlapping setsof interdigital electrodes, each set being connected to a correspondingone of two opposite bus bars. The strength of each element is a functionmainly of the length at right angles to the acoustic surface wavepropagation direction, of said overlap, although end effects and theeffects of nearest neighbouring electrodes have also to be taken intoaccount depending on the transducing magnitude of the element (sometimesreferred to as the strength of the source) under consideration. Thedistribution of overlaps along an apodized transducer can convenientlybe indicated in a diagrammatic representation by the overlap envelope,which comprises a pair of lines drawn to follow the respective ends ofthe overlap between adjacent electrodes along the interdigital array.

In order to reduce the size of an acoustic surface wave device operatingat a given frequency an attempt has been made to reduce the size of theinterdigital transducer array. In general the length of the array willbe determined by design factors such as the bandwidth and rejectioncharacteristics required. However, when the width of an interdigitalarray is reduced, various difficulties tend to arise. For example,adverse diffraction effects will increase, also insertion loss will tendto rise necessitating the use of a crystal orientation having arelatively high coupling factor.

Acoustic surface wave transducers are frequently arranged on ananisotropic piezoelectric crystal so as to direct acoustic surface wavesas nearly as possible along a self-collimating direction, i.e. adirection for which the velocity is a minimum relative to that fordirections inclined to either side, so that if a diffracted acousticsurface wave departs from the said direction it will tend to travelfaster and at least some of the acoustic surface wave energy will beturned back into the desired direction. Such a self-collimatingdirection can sometimes be associated with a lower coupling factor.

When a direction is used in which the self-collimating effect is reducedor absent, for any reason, such as to provide a higher coupling factor,notably to reduce insertion loss when the array is made narrower, theeffects of diffraction on that part of the acoustic surface wave whichtravels furthest through the array will tend to become unacceptable,notably in an apodized structure in which the shorter overlap elementstend to occur near the ends of the array, causing the response of thedevice to depart significantly from the designed response.

Thus, as the width of an interdigital array is reduced under theseconditions, acoustic surface waves which depart slightly from the axialdirection in either direction as a result of diffraction will tend to bereflected back by the propagation discontinuity formed by the outerboundaries of the pair of bus bars due to the change in surface loading,and to propagate along the transducer as a multi-mode guided wave eachmode of which has a phase velocity and a group velocity respectivelyfaster and slower than the axial propagation velocity of that part ofthe acoustic surface wave which is propagating with a planar wavefrontset at right angles to the axial direction. The effect of multimodingcauses the measured transducer characteristic to depart increasinglyfrom the design characteristic as the width of the array is reduced to afew wavelengths, e.g. into the region 12λ_(c) to 7λ_(c).

It is an object of the invention to provide an improved acoustic surfacewave device of compact construction which can ameliorate some or all ofthe aforesaid difficulties.

According to the invention there is provided an acoustic surface wavedevice including a piezoelectric substrate for propagating acousticsurface waves at a surface thereof and an acoustic surface wavetransducer formed on said surface to launch or to receive acousticsurface waves at said surface along a propagation direction parallel tothe axis of the transducer. The said transducer includes at least oneinterdigital array of two overlapping sets of electrodes and each set isconnected to one of a pair of opposite bus bars. The invention ischaracterized in that the overall width of the or each interdigitalarray between the outer boundaries of the or a said pair of bus bars,measured in a direction at right angles to the acoustic surface wavepropagation direction, is so determined in relation to the wavelengthλ_(c) at the center frequency f_(c) of the acoustic surface wavepass-band of the transducer, and in relation to the electrodedistribution, that the or each said interdigital array functions as anacoustic surface waveguide which will only propagate and transduce asingle acoustic surface wave guided energy mode which is symmetricalabout the central propagation axis of the array.

The invention is based on the realization that by reducing the width ofthe array still further, it is possible to construct an interdigitalarray between a pair of opposing bus bars, which not only operatesprincipally in a waveguide mode but will in fact only support a singlesymmetrical propagation mode, so that a weighted array can beconstructed in which the problem of diffraction from small transducingelements (sources) at the far end of the array can be overcome, and inwhich, if desired, a relatively small bandwidth can be provided by along but narrow array, again with substantially less disturbance fromdiffraction effects.

In the case of certain orientations of the propagation surface onlithium niobate, such as the Y-cut Z-propagating, or 124°, 128° or 131°rotated Y-cut X-propagating configurations, the overall width of the oreach interdigital array is preferably not greater than 3.5λ_(c),although the width may be increased to not greater than 5λ_(c) providedthat the interdigital array is made sufficiently symmetrical about thecentral propagation axis to ensure that only a single energy mode whichis symmetrical about that axis can be transduced thereby. It should benoted however that the optimum width for single symmetrical modepropagation will depend, inter alia, on the coupling factor, a highercoupling factor in general leading to a narrower width. The interdigitalarray can be apodized if desired.

The guiding effect of the interdigital array enables an anisotropicpiezoelectric crystal to be used, if desired, with the crystal surfaceoriented so that the acoustic surface wave propagation direction doesnot lie along a self-collimating direction for said propagation.

Preferably the substrate is Y-rotated X propagating lithium niobate inwhich the Y-rotation of the cut lies in the range from about 124° to128° so that the undesired effects of bulk waves can be optimallyreduced.

Embodiments of the invention will now be described by way of example,with reference to the accompanying drawings, in which:

FIG. 1 shows an acoustic surface wave filter embodying the invention,

FIG. 2 shows a portion of an apodized transducer array in accordancewith the invention,

FIG. 3 shows a portion of a uniform transducer array in accordance withthe invention, and

FIG. 4 is a graph illustrating the amplitude profile across a transduceraxis.

In FIG. 1 an acoustic surface wave device embodying the invention, inthe form of a band-pass filter, comprises a piezoelectric substrate 1for propagating an acoustic surface wave at a surface thereof,transducer means comprising an apodized input transducer 2 for launchingacoustic wave energy into a propagation track 11 at said surface and auniform output transducer 3 for converting acoustic wave energypropagating along the track 11 into an electrical signal. The input andoutput transducers are both of the interdigital type, the electricalinput signal being applied between the two sets of electrodes of thetransducer 2 and the electrical output signal being taken off fromacross the two sets of electrodes of the transducer 3. The overlapenvelopes of the interdigital electrodes of the transducers 2 and 3 areshown respectively by the lines 8 and 9. Since the interdigitaltransducers 2, 3 are bidirectional, the unwanted surface wave energylaunched in a direction away from the end which communicates with theother transducer 2, 3, is absorbed at least partially, by dampingmaterial 10 applied to the surface between the end of the transducer 2and the edge 6 and, for corresponding reasons, between the end of thetransducer 3 and the edge 7, and to other parts of the surface whereundesired acoustic wave energy can propagate with the exception of theactive regions of the interdigital arrays. To this point in thedescription the device corresponds to the device described andillustrated in FIG. 3.7 on page 71 of the aforesaid book, AcousticSurface Waves edited by A. A. Oliner.

In accordance with the invention the overall width of each of theinterdigital transducer arrays 2 and 3 between the outer boundaries ofthe corresponding pair of bus bars, e.g. 14 and 15 in the transducer 2shown in part in FIG. 2, measured in a direction at right angles to theacoustic surface wave propagation direction, must be so determined inrelation to the wavelength λ_(c) at the center frequency f_(c) of thepass-band and in relation to the electrode distribution, that each ofthe transducer arrays 2, 3 functions as an acoustic surface waveguidewhich will only propagate and transduce a single energy mode which issymmetrical about the central propagation axis (20 in FIG. 2) of thearray.

FIG. 4 illustrates the amplitude profile produced across the symmetricalsingle guided mode acoustic surface wave beam propagating in thetransducers 2 and 3 in accordance with the invention and shows the waveamplitude a plotted against the lateral distance Z from the axis 20 ofthe transducer measured in wavelengths (λ) for a 3λ transducer on 124°Y-rotated X-propagating lithium niobate. The amplitude variescosinusoidally about the transducer axis 20 and within the metallizedregion 22, i.e. within the boundaries of the interdigital transducerpattern including the bus bars, and then tends to fall awayexponentially beyond that region.

This profile is independent of any transducer element weighting producedby altering the amount of electrode overlap because in a transducer inaccordance with the invention only this one (symmetrical) mode canpropagate. Effective weighting can, however, be achieved by apodizationbecause the position in the Z-direction with respect to the transduceraxis 20, of the break in an electrode at which the polarity reverses,determines the strength of coupling to the guided mode. The weight ofthe ith transducing element (source) is to a first order proportional tosin (kZ_(i))/k (where k is a crystal parameter), instead of beingproportional to Z_(i) as in the conventional wide aperture transducer,where Z_(i) is the distance between the break in the electrode and thecenter line (axis 20) of the structure. Because the beam profile remainsconstant, apart from a very small overall variation with frequency whichis dependent only on the width of the guide and not on the weighting,the frequency response of the filter is essentially the product of theresponse of the two transducers, and conventional synthesis can be usedto determine the weights required to meet a given specification. Thesource strength correction hereinbefore mentioned can be readily appliedto the final geometrical implementation of the transducer electrodepattern. The parameter k is mainly a function of the crystal orientationand guide width, and only varies very slowly with frequency and canprobably be regarded as a constant.

The transducer array 2 of FIG. 1 is shown in part, enlarged, in FIG. 2,and comprises an apodized interdigital array of two sets of electrodesin which the electrodes of each set are connected to a corresponding oneof the pair of opposite bus bars 14, 15, having respective terminalconnections 18, 19 (FIG. 1). The bus bar 14 is connected to a source ofsignal and, in order to minimize capacitive breakthrough, the bus bar 15is connected to signal ground. The respective sets of electrodes andcorresponding bus bars can however, if desired, be driven in antiphaseby a source of signal voltage, but in that case a further conductivematerial pattern connected to ground may be required adjacent the busbars as a screen to reduce direct electrical breakthrough. A groundedscreen 5 formed conventionally by metallization is interspersed betweenthe two transducers 2 and 3, and to assist the guided wave it is alsomade a suitable width so as to support the single symmetrical guidedmode.

The electrodes forming each of the two sets of the apodized interdigitalarray 2 suitably comprise double electrodes 24, 25, as described in apaper entitled "Applications of double electrodes in acoustic surfacewave device design" by T. W. Bristol et al. presented to the Proc IEEEUltrasonics Symposium, October 1972, in order to reduce adversereflection effects from the electrodes. The spaces between theelectrodes of a given set not occupied by overlapping portions ofelectrodes of the other set are filled by dummy electrodes 21, alsodouble, connected to the same bus bars as the electrodes of the givenset, as described in Applied Physics Letters, 1st December, 1971, Volume19, Number 11, at pages 456 to 459.

The communicating end 13 of the transducer 2 is arranged as described inEuropean Patent Application No. 83200550.8, so that the undesiredtransducing edge-element (edge source) at the outer boundary of thesignal-driven end-electrode or bus bar will direct or receive acousticsurface waves along a direction away from the array propagation axis 20.

In forming an interdigital array using double electrodes as in FIG. 2,the periodicity of the double electrodes will be λ/4 and the width ofeach electrode will be λ/8. It must be understood, however, that thepresent array 2 is formed with an overall width such that acousticsurface waves propagate therealong in a single symmetrical guided mode.Such a mode is formed, in accordance with waveguide theory, by thesuperposition of synchronised intersecting acoustic surface wavefrontsreflected at or near the outer boundaries. Thus the guided acousticsurface wave mode will propagate along the array with a phase velocitygreater than the normal surface wave propagation velocity in a widearray, but the acoustic surface wave energy will only travel along thearray at the group velocity of the mode which will be less than the freepropagation velocity. Consequently the electrode size and spacing mustcorrespond to the phase velocity, while the magnitudes of thetransducing elements (sources) formed by pairs of overlapping doubleelectrodes must be related in the impulse-time domain to the positionsof those overlaps with respect to the group velocity of the guided mode.

The uniform transducer 3, shown enlarged and in part in FIG. 3, is alsoformed using double electrodes 34, 35, connected in a similar manner tothat of transducer 2 shown in FIG. 2, between opposing bus bars 31, 32,and the ends of the array in the vicinity of the inwardly inclinedportions 31', 31", 32', 32" of the bus bars 31, 32 is filled with dummyelectrodes 22.

The communicating end 36 of the transducer 3 is formed in a mannersimilar to the end 13 of the transducer 2 in order to reduce the effectsof undesired end transducing elements (end sources). The overall widthof the array 3 between the outer boundaries of the opposing bus bars 31,32 is, as in the case of the transducer 2, such that acoustic surfacewaves propagate therealong in a single symmetrical guided mode.

As shown in FIG. 1, the apodization of the launching array 2, indicatedby the envelope 8, is in the form of a main lobe and minor half lobes ateach end. This relates to a particular example of a device according tothe invention, and other forms of apodization patterns can be used asdesired. In fact other forms of weighting can be employed or noweighting at all, provided the array will propagate and transduce asingle acoustic surface wave guided-energy mode which is symmetricalabout the central propagation axis of the array.

It should be noted that the guided energy mode will in fact extend ashort distance beyond the outer boundaries of the bus bars so care mustbe exercised in applying the damping medium 10 to the propagationsurface of the device to ensure that a sufficient region is kept clearto either side of each transducer.

Since the end portions 13, 36, of both arrays are grounded, the screen 5can be omitted and the two transducers brought adjacent to one another.

The interdigital electrodes of the embodiment can be formed on thepiezoelectric substrate in conventional manner by a process ofphotolithography. The substrate can conveniently be Y-cut Z-propagatingor 124° to 131° Y-rotated X-propagating lithium niobate on which thetransducer arrays can each have a width between the outer edges of thebus bars lying in the range 3λ_(c) to 3.5λ_(c). In one example, theoverall width was 3.2λ_(c) and the width of the electrode array betweenthe bus bars was 1.8λ_(c). The overall width can be increased to notmore than about 5λ_(c) if the array is made symmetrical about thepropagation axis. If a substrate having a higher coupling coefficient isused, for example 41° Y-rotated X-propagating lithium niobate, theoverall width of the array would have to be reduced correspondingly toensure operation in the single symmetrical guided propagation mode.

One practical application of the device described is in the manufactureof a television receiver intermediate frequency filter and wouldpreferably take the in-line form of FIG. 1, the device being formed on124° to 131° Y-rotated X-propagating lithium niobate. Although theinsertion loss of such a device would tend to be greater than that of asurface acoustic wave filter of conventional wide aperture, theimpedance of the transducers would be greater so that a device inaccordance with the invention can be connected directly into a circuitwithout the need for the series connected matching resistor frequentlyused with wide aperture devices. Consequently the overall insertion lossmay have a similar value in the two cases.

We claim:
 1. An acoustic surface wave device comprising a piezoelectricsubstrate for propagating acoustic surface waves at a surface thereofand an acoustic surface wave transducer formed on said surface to launchor to receive acoustic surface waves at said surface along a propagationdirection parallel to the axis of the transducer, in which saidtransducer includes at least one interdigital array of two overlappingsets of electrodes with each set connected to one of a pair of oppositebus bars, characterized in that the overall width of an interdigitalarray between the outer boundaries of a said pair of bus bars measuredin a direction at right angles to the acoustic surface wave propagationdirection is so determined, in relation to the wavelength λ_(c) at thecenter frequency f_(c) of the acoustic surface wave pass-band of thetransducer, and in relation to the electrode distribution, that a saidinterdigital array functions as an acoustic surface waveguide which willonly propagate and transduce a single acoustic surface wave guidedenergy mode which is symmetrical about the central propagation axis ofthe array.
 2. A device as claimed in claim 1, wherein the substratecomprises Y-cut Z-propagating or 124° to 131° rotated Y-cutX-propagating lithium niobate, characterized in that said overall widthof an interdigital array lies in the range 3λ_(c) to 3.5λ_(c).
 3. Adevice as claimed in claim 1, wherein the substrate comprises Y-cutZ-propagating or 124° to 131° rotated Y-cut X-propagating lithiumniobate, characterized in that said interdigital array is symmetricalabout the central propagation axis thereof, and said overall width ofthe array is not greater than 5λ_(c).
 4. A device as claimed in claim 1,characterized in that the piezoelectric substrate comprises ananisotropic crystal oriented so that said propagation direction does notlie along a self-collimating direction for the propagation of acousticsurface waves.
 5. A device as claimed in claim 1, characterized in thata said interdigital array comprises an apodized array.
 6. A device asclaimed in claim 2 wherein the piezoelectric substrate comprises ananisotropic crystal oriented so that said propagation direction does notlie along a self-collimating direction for the propagation of acousticsurface waves.
 7. A device as claimed in claim 3 wherein thepiezoelectric substrate comprises an anisotropic crystal oriented sothat said propagation direction does not lie along a self-collimatingdirection for the propagation of acoustic surface waves.